A tension leg platform, or TLP, is a marine platform having economic potential for use in drilling in deep water locations. A typical tension leg platform is similar to a semi-submersible platform by having a number of large, vertical cylinders close to the periphery for providing stability during the transportation phase from shore to the drill site. The platform has a lower structure tieing the cYlinders together at the bottom thereof in order to provide some buoyancy. It also has an upper structure tieing the upper ends together, and providing space for equipment, supplies and the like.
At the drill site, the tension leg platform is connected by a plurality of tendons to a heavy anchor secured on the sea bottom. The tendons are vertically oriented, and are always under tension. The tendons are quite slender and flexible, and act essentially as strings to permit the tension leg platform to move relatively freely in the horizontal plane while substantially preventing upward and angular movement. The tension leg platform provides considerable advantage over a semi-submersible platform, because the wellhead may be above water and drilling and work over may be done in the conventional surface fashion.
The hydrodynamic behavior of a tension leg platform has certain characteristics during the construction and installation phase which are at odds with the characteristics desired in the on-location phase. These characteristics impose conflicting design requirements. When on-location, it is desirable to reduce the vertical wave forces as much as possible, and thereby to reduce the strength, weight, number and cost of the tendons and the anchor. In order for this to be done, the buoyancy should be well below the surface of the water. Thus the cylindrical columns should be as deep in the water and as small in diameter as possible. Such a configuration, however, results in a very tall structure which is unstable when floating to location, and also has excessive draft during construction. Stability while floating to location is an absolute requirement, with the result that the goal of minimizing the on-location wave forces is normally sacrificed.
A further limitation on the prior art tension leg platform has to do with the buoyancy effect resulting from wave motion. Passage of a wave beyond a column causes that portion of the column normally disposed above the sea surface to be covered, thereby increasing the buoyancy thereof. Those portions normally disposed below the sea surface, on the other hand, suffer from reduced buoyancy when this occurs, on account of the motion of the water particles resulting from the cyclic nature of the wave. The net result is that the buoyancy may be adversely affected by wave action, particularly by waves of substantial amplitude.
The disclosed invention is one which overcomes the conflicting design requirements normally presented by a tension leg platform. The disclosed invention combines the advantages of a jack-up drilling platform with those of a tension leg platform in order to provide a mobile marine platform which has a first geometry during the construction phase and a second geometry during the on-location phase. In the first geometry the work platform is closely spaced to a pontoon system floating on the surface of the water and with the pontoon system substantially providing stability because no tendons are connected. In the second geometry, the work platform is spaced a substantial distance above the pontoon on a column carried by the pontoon and stability is substantially provided by the tendons connected to the anchor. The dimensions resulting from this two geometry system permit columns that are much smaller in diameter and extend to a much deeper operating draft than heretofore possible.
A further advantage of the invention involves the relationship between the buoyancy provided by the column and that provided by the pontoon. It is theoretically possible to size the two elements so that the theoretical wave forces cancel. The disclosed invention permits such dimensional configurations to be more easily taken into account, with the result that full advantage can be obtained of this phenomenon.